The proposed studies will provide new information about the mechanism of active potassium transport by the midgut epithelium of larval insects. This epithelium is thought to be a model for understanding potassium transport as it occurs in a variety of insect tissues. Specifically, we plan to characterize the process whereby potassium enters the transporting cells, the effect of this basal potassium entry step in the process of active transepithelial potassium transport and regulation of intracellular potassium, and the role of transcellular and paracellular shunt pathways for solute movement in determining the transport properties of the tissue. Two basic experimental approaches will be used - intracellular potassium - and voltage-sensitive electrodes will be used to measure the electrochemical gradient accompanying potassium uptake, and isotopic techniques will be used to measure the rate of uptake by transporting cells, the conductance of epithelial shunt pathways, and the electromotive force developed by the potassium pump. The results of this study will add to our knowledge of the mechanisms of salt transport by epithelia, and may reveal features of insect physiology important for the development of more specific insect control agents.